Gypsum plaster and process of making the same



Patented Apr. 23, 1935 1,9h9d58' GYPSUM rLAs'rEa AND raoouss or Maniac rns same canine K. Boos, Wheaton, llll., assignor to United States Gypsum Company, Chicago, 111., a corporation of illinois .No Drawing. Application October 13, 1932, Serial No. 637,673

. 11 Claims. (Cl. Elfi -34) The present invention relates to a stabilized somewhat undesirable for reasons which will be plastic-working calcined gypsum wall plaster and mentioned hereinbelow. The ordinary C m e the process for its manufacture, the main object cial gypsum wall plaster, when freshly manufacbeing the production of a stronger and more, tured, is found to have a setting time of about stable product. Y i five hours, particularly when it ls employed in 5 One of the objects of the invention is to treat admixture with sand in the ratio of two parts of pre-aged calcined gypsum which ordinarily has a sand to one part of the gypsum plaster, both by comparatively low consistency, so as to increase weight. Such a mixture, howevenis not stable, its consistency as well as its plasticity; while at but after aging for ashort time, for example four the same time rendering the product thus proweeks, will have a setting time of eight hours, and 10 duced more stable as to its setting time than simiafter a further storage of two months will be lar products of the prior art. found to require as much as twelve hours or A further object of the invention is to treat lon er to set. After two months have elapsedfithe Dre-aged calcined gypsum by mechanical means, rate of increase in setting time is slower, for by 15 such for example as grinding, so as to increase the that time most of the solubleanhydrite will have 15 plasticity thereof and to render its setting time been converted into hemihydrate so that the comless subject to change upon storage. mercial gypsum plaster then will set in from six- A further object of the invention is to grind teen to twenty hours. Further St ag h w v pre-aged calcined gypsum, either retarded or un-, has the opposite effect that is to say, during the retarded, to a degree of fineness sufficient subsecond year sufficient moisture will have been ab- 20 stantially to increase its plasticity and consistency sorbed. by the hemihydrate to produce a few while at the same time producing a product whose minute seed crystals of calcium sulphate 'dihysetting time does not change materially upon drate which, because of their catalytic efiect, renlapse of time. 1 der the product quick setting. Eventually the i Further objects of the invention will become product will be so greatly accelerated in setting- 25 apparent from the detailed description hereintime by the presence of these crystals that it will below and from the claims hereunto appended; set as fast as from five to ten minutes after being In order that a better understanding of the mixed with the gaging water. present invention may be had, the properties of Obviously such changes are. extremely undesirthe prior art products upon which this invention able, a the p op fi 0f the commercial product 30 a great improvement will be briefly considered. will be depe p its e and t W l th e- For example, ordinary ground calcined gypsum or fore require a preliminary test. before the user plaster of Paris such as is used as a base for the can be a u d o how it Will act When pp production of gypsum wall plasters, consists of a to the wall. A further undesirable result of the 5 more or less heterogeneous mixture of different formation of hemihydrate from the soluble anhyphases including predominantly calcium sulphate drite and the formation of the aforesaid seed hemihydrate and anhydrous calcium sulphate of crystals is the concomitant lowering of the the variety known as soluble anhydrite. Such-a waterzplaster ratio for standard working consistcommercial calcined product, in order to constiency. Simultaneously with the lowering of the 4 tute a usable wall plaster is usually furnished to consistency there occurs a loss of plasticity and the trade in admixture with small quantities of the ability ofthe plaster to producea sufficiently filamentous material, such as hair or equivalent plastic mortar when mixed with the usual three fiber, these added materials being employed in or four parts of sand, such as is used in common amounts of from 4 to 10 lbs. per ton of plaster; wall plastering operations.

In order to render such commercial plasters slow These changes in the setting time and loss of 45 setting, from 6.25% to 0.5% of organic colloidal plasticity are largely the result of the instability materials are added, the most common being of the combination of mixed phases present in some form of glue or similar animal product. It ordinary calcined gypsum. The reason for this is a characteristic of ordinary commercial gypsum is that the efiect of retarders is different upon the wall plaster to absorb some moisture during storsetting time of the soluble anhydrite than it is 50 age, this resulting in the hydration of the therein upon that of calcium sulphate hemihydrate. For contained soluble anhydrite, thereioy converting example, soluble anhydrite requires about 50% the same into the hemihydrate. The total effect more retarder than does the hemihydrate to proof this waterabscrpticn is to renderthe plaster duce a product having the same setting time.

much slower Setting and also, to make the same Therefore a freshly made calcined gypsum wall 55 Wires sr @FFICE plaster properly retarded by means of commercial retarders will be satisfactory when first made, but later on as the soluble anhydrite disappears, the product will be greatly over-retarded. Furthermore, the soluble anhydrite requires more water to produce standard consistency than does the hemihydrate and consequently as the soluble anhydrite disappears the amount of gaging water required to bring about a gaged product or" standard consistency is decreased. Now inasmuch asthe amount of retarder in the plaster remains constant but less water is added the resultant solution of the retarder in the water is more concentrated and hence the retarding effect is enhanced, thereby causing the product to have a longer setting time than is desirable.

The aging effects found in the prior art commercially retarded gypsum wall plasters are further complicated by the fact that some crystal and particle accretion occurs during these changes in phase, thereby further lowering the amount of gaging water required to bring the plaster to standard consistency. For instance, ordinary calcined gypsum of average purity when freshly manufactured will require from 75 to parts of waterper 109 parts-oi the plaster by weight to produce a mix of standard consistency. After such a product has stood for six weeks, however, under ordinary storage conditions, it will be found that it requires only 65 parts of water. Three weeks later its consistency will be as low as 60, and will then during the ensuing six months drop to as low as 56, remaining practically sta- V tionary thereafter.

The above defects are overcome by the practice f the present invention. It has now been discovered by the inventor that calcined gypsum which is composed largely of hemihydrate, and

particularly such a product as is produced by artificial or pro-aging methods during calcination, may be retarded with commercial or organic retarder with the result that the plaster so made remains relatively stable over a period of months. However, such a type or calcined gypsum has heretofore been used exclusively for such purposes where quick setting is essential, namely in ornamental molding and for. industrial casting purposes. When so used the product is moder ately fine ground and employed without the addition of retarders. However, such types of plaster are notordinarily used, nor are they suitable for plastering purposes, used either alone or in admixture or the same, for the reason that they lack th necessary plasticity and do not work well when mixed with the usual quantity of sand such as is employed for plastering purposes. As a result the yardage covered per ton of material is much lower than is the case with ordinary commercial gypsum wall plaster.

A pro-aged calcined gypsum plaster such, for example, as that described in the United States Patent No. 1,370,581, has a sand carrying capacity of about two'and one-half to three parts by weight of plaster. In accordance with the present invention, however, such a type of plaster can be made to work plastic and satisfactorily with as much as four parts oi sand by weight, by the expedient of grinding the same superfine, as for example in an open circuit tube mill with a feeding ratio or" about 1 to 8 of the grinding media, by weight. In such grinding it has been found unnecessary to take any special precaution to restrain the liberation of moisture or to cause it to recombine with the ground product, as is for examble described in United States Patent No.

1,392,574. Therefore, in practicing the invention the gypsum is calcined so as to leave therein the theoretical proportion of, moisture required to constitute calcium sulphate hemihydrate or slightly more but not to exceed 0.5% above the theoretical requirement as based upon the calcium sulphate content of the product; that is to say, the product initially should contain from 6.2% to 6.7% of water. By allowing this amount of moisture to remain, the production of any appreciable quantity of soluble anhydrite is avoided. The sheet, furthermore, can be explained by the fact that the plaster-as fedto the tube mill contains relatively little of the soluble anhydrite and therefore, since the hemihydrate predominates as the more stable of the two phases,.it will serve to repress the tendency to form soluble anhydrite.

In the practice of' the present invention a preaged low consistency calcined gypsum plaster such as is described in Brookby Patent No. 1,870,581 is employed. The product of said'patent contains a small amount of a deliquescent salt of an alkaline earth" metal, such for example as calcium chloride. Alternatively, a ypsum plaster that has been artificially aged by means of steam orwater'vapor, or by other means,'may

be employed. The fineness of such pro-aged gypsum plaster is of such a degree that ordinarily from75 to by weight'of the product will pass through a standard mesh screen. Sacha plaster contains little or no soluble anhydrite and requires to bring it to standard consistency from 50 to 60 parts by weight of water to 10L- parts by weight of plaster. Also, if such a product is ground as fine as ordinary plaster it lacks the required plasticity. By subjecting such a plaster, however, to a superfine milling process as herein below described, it will be rendered plastic and capable of carrying upwards of four parts weight of sand.

The amount of continuous grinding necessary will vary with the different plasters employed. For instance, some of the purer plasters are quitesoft and hence require much less work for reduction than do the denser and less pure variety.

For example, if pro-aged plaster of average purity,

ured before the superfine grinding is increased by at least 10 partsper 100 parts of plaster by weight it will be found to have acquired suflicient plasticity and sand-carrying capacity to come within the scope of the present invention. In carrying out the process, thereiorc, such a. pre-aged plaster is ground sufficiently to increase its consistency by from 15 to 29 parts, but not to exceed 25 partsabove its original consistency. Preferably a pro-aged calcined gypsum whose standard consistency prior to grinding is 56 is ground for a sufricient length of time so that after leaving the mill it will require '75 parts of water to parts by weight of plaster. This ground product is then mixed. with the usual amount of fiber and a sufiicient quantity of a commercial organic retarder to give it the desired setting time. 1

The following comparison of the relative sta bility of prior art commercial retarded gypsum Cir to its: stability of setting time with the ordinary gypsum'wall plasterv manufactured from thesame source and: retarded to. produce the same initial fresh setting time, thetest being made with two parts of standard testing sand to one part of the respective plasters, the results would be as follows:

The improved plasticity obtained is termed mechanical plasticity, that is to say, while a large percentage of the particles are reduced to ten micron diameter or smaller, they are still stable hemihydrate and as they have not been dehydrated by the grinding operation, they approach a semi-colloidal condition merely by subdivision.

Calcined gypsum wall plaster manufactured by the process of the present invention will carry from four to five parts of sand by weight without the necessity of using the excessively high additions of gauging water required ordinarily for freshly manufactured wall plasters with high soluble anhydrite content. As a result, one of the advantages of the new process is the production of a stronger and better plastered wall. Another advantage of the plaster produced by the present process is that its sand-carrying capacity after one year is nearly equal to that when freshly made, whereas most other plasters usually lose 50% in sand-carrying capacity within that time. The amount of water required for standard consistency drops twenty to twenty-five parts per hundred parts of wall plaster by weight for ordinary plasters in one year, whereas wall plaster made by the present process is lowered only from five to ten parts per hundred. A further and most important result obtained by the process is the improvement in the stability of the setting time.

To sum up the process, therefore, it comprises grinding pre-aged low consistency commercial calcined gypsum plaster so as to raise its con sistency as well as its plasticity and to stabilize the setting time so that it will remain substantially constant after having reached its maximum.

There are no mechanical limitations on the present invention further than that the grinding is preferably carried out in a tube mill. The time of grinding must be determined for each particular type of plaster and this can readily be accomplished by taking a sample thereof and testing it for consistency and comparing the consistency thus found with that of the product prior to the grinding operation. The present invention has been found to be extremely useful for it enables the commercial production of gypsum plasters whose characteristics are definitely fixed so that they may be used with the utmost assurance of constant results and without the necessity of experimentation with each new lot of the product purchased.

I would state in conclusion that while the de scribed example constitutes a practical embodiment of my invention, I do not wish to limit myself precisely to these details, since manifestly the same may be considerably varied without departing from thespirit of the invention as defined in the appended claims.

- Having thus describcdmy invention, I claim as new anddesire tosecure by Letters'Patent:

1. A calcined gypsum wall plaster made from artificially aged'calcined gypsum carrying a small amount of water-soluble deliquescent substance intimately incorporated therewith, being substantially free from soluble anhydrite, and having a testing consistency ranging between sixtyfive and seventy-five parts of water per one hundred parts of neat plaster by weight.

2. A set-stabilized retarded calcined gypsum wall plaster made from a low-consistency calcined gypsum carrying a small amount of water-soluble deliquescent substance intimately incorporated therewith, rendered plastic by regrinding.

3. The process of producing a relatively stable plastic gypsum wall plaster which comprises regrinding calcined gypsum, carrying a small amount of water-soluble deliquescent substance intimately incorporated therewith, suificiently to raise the water required for testing consistency ten to twenty parts per one hundred parts of plaster by weight above the amount of water required per hundred parts of plaster before regrinding.

4. The process of stabilizing the setting-time of low-con istency gypsum wall plaster containing a small amount. of a deliquescent substance incorporated therewith which comprises regrinding said plaster under such conditions as to avoid the production of soluble anhydrite from the calcined gypsum therein contained.

5. The process of producing plastic gypsum wall plaster that is stabilized as to its setting-time which comprises regrinding calcined gypsum, containing incorporated therewith a small amount of a water-soluble deliquescent substance, without appreciably changing its moisture con.- tent, for a sufiicient length of time so as to increase the water required for bringing the reground product to testing consistency ten to twenty parts per hundred parts of plaster, by weight, above its original water-requirements.

6. A retarded calcined gypsum wall plaster made from preaged calcined gypsum carrying a small amount of water-soluble deliquescent substance intimately incorporated therewith, rendered plastic by regrinding.

7. A retarded calcined gypsum wall plaster made from preaged calcined gypsum carrying a small amount of water-soluble deliquescent substance intimately incorporated therewith, rendered plastic by tube-mill grinding.

8. A retarded calcined gypsum wall plaster artificially aged by incorporation therewith of a deliquescent water-soluble substance, made plastic by regrinding.

9. A retarded calcined gypsum wall plaster stable as to its setting time and consisting of gypsum plaster artificially aged by incorporation therewith during its manufacture of a small amount of a deliquescent salt of an alkaline-earth metal, made plastic by regrinding.

if). A highly plastic calcium sulphate hemihydrate plaster having a setting time, plasticity, and sand-carrying capacity, none of which properties is materially affected by length of time of storage, consisting of retarded gypsum plaster containing a small amount of calcium chloride, and. reground so as to be rendered plastic and to have a testing consistency of from ten to twenty parts greater than that of a similarly constituted plaster that has not been reground.

11. The process of stabilizing the setting-time the mixture for a length of time sufficient to inand plasticity of a calcium sulfate hemihydrate crease its testing consistency ten to twenty parts plaster against changes due to lapse of time, above the normal testing consistency of the said which comprises incorporating with calcium sulcalcium sulfate hemihydrate.

5 phate hemihydrate a small amount of a deliquescent Water-soluble substance and. regrinoling CARLISLE K. ROOS. 

